1. Field of the Invention
The present invention relates to cassette tape recorders, such as video tape recorders and audio tape recorders, for recording signals on a magnetic tape wound on a pair of reels in a tape cassette or reproducing signals from the magnetic tape, and more particularly to a control system for moving the magnetic tape at a high speed and winding the tape on one of the reels.
2. Description of the Related Art
VTRs (video tape recorders) are conventionally settable in a fast-forward (FF) mode and rewinding (REW) mode. In these modes (hereinafter referred to as xe2x80x9chigh-speed winding modexe2x80x9d), the rotation of a capstan motor is delivered to the reel on the take-up side to transport the magnetic tape at a high speed. In recent years, a high-speed winding mode is realized in which the tape speed is 300 to 400 times the playback speed.
With reference to FIG. 10, provided in such a high-speed winding mode are an acceleration section R1, constant-speed section R2, deceleration section R3 and low-speed section R4, as arranged from a position where the tape is started for a high-speed movement to the tape tail end. In the constant-speed section R2, the capstan motor rotates at a maximum speed, giving a maximum tape speed Vm. The tape is thereafter wound to a predetermined tape position Pb, whereupon the tape starts to decelerate. The tape speed then decreases to a predetermined value Ve, whereupon the tape is brought into a slow-speed movement and wound up to the winding end.
The tape position Pb for the transition from the constant-speed section R2 to the deceleration section R3 is detected by a known method of detecting the remaining amount of the tape (see, for example, JP-A No. 81389/1990). Stated more specifically with reference to FIG. 11, suppose the radius of a magnetic tape 17 wound on a supply reel 15 of a cassette 1 is Rs, the radius of the tape 17 wound on a take-up reel 16 is Rt, and the radius of each of the supply reel 15 and the take-up reel 16 is r. It is possible to calculate from these values the area of the tape 17 wound on the supply reel 15, i.e., the area Ss between the circle with the radius r and the circle with the radius Rs, and the area of the tape 17 wound on the take-up reel 16, i.e., the area St between the circle with the radius r and the circle with the radius Rt. Further the radii Rs, Rt of the tape 17 on the respective reels can be calculated from the speed V of the tape 17 and the rotational periods Ts, Tt of the reels. The reel radius r and the tape speed V are known values.
For example, in the case where the tape is transported from the supply reel 15 to the take-up reel 16 and assuming that the tape 17 has an entire length Q, the amount Lm of the tape 17 remaining on the supply reel 15 can be calculated from the following mathematical expression.
Lm=Qxc2x7Ss/(Ss+St)
However, the thickness of magnetic tapes for use in tape cassettes for conventional VTRs involves variations (e.g., 17 xcexcm to 20 xcexcm) due to the tolerance and difference of manufacturer, consequently impairing the accuracy in measuring the amount of remaining tape. Even if the tape is wound on the reel on the supply side over the same area Ss, the actual length of the tape portion providing the area is short when the tape has a large thickness, or is large when the tape has a small thickness.
In the case where the tape moving at the constant speed is decelerated as seen in FIG. 12 based on such inaccurate detection of the amount of remaining tape, there arises the problem that if the tape thickness is small, the actual deceleration starting position is shifted to a position Pbxe2x80x2 from the intended tape position Pb toward the supply side, with the result that the length of the tape to be wound up to the tail end by the low-speed movement increases to a value R4xe2x80x2 which is greater than the minimum length R4 required, necessitating a longer period of time for winding up the tape to the trailing end.
Alternatively if the tape has a large thickness, the actual deceleration starting position is shifted to a position Pbxe2x80x3 from the contemplated tape position Pb toward the take-up side, with the result that the tape is wound up to its trailing end at a great speed Vexe2x80x3 during deceleration before the movement at the low speed Ve. This entails the likelihood that the resulting impact will damage the tape.
Further in the constant-speed section, the capstan motor is out of speed control, rotating at the greatest possible speed, so that differences in the magnitude of the load involved at this time produce variations in the tape speed. For example, in the case where the tape is moved at a constant speed Vmxe2x80x2 which is less than the specified speed Vm as shown in FIG. 13 and if the tape is then brought into a decelerated movement at the predetermined tape position Pb, the low-speed section is subsequently reached earlier, with the result that the tape portion to be wound up by the low-speed movement has a length R4xe2x80x2 which is greater than the minimum length R4 required, leading to the problem that the winding of the tape up to its trailing end requires a longer period of time.
The problems described above become more pronounced when the high-speed winding mode is realized at a tape speed at least 500 times the playback speed.
Accordingly, an object of the present invention is to shorten the time required for winding up a magnetic tape by giving specified speed variations to the tape in the high-speed winding mode even when the tape thickness varies from tape to tape.
The present invention provides a cassette tape recorder which comprises a tape transport mechanism for unwinding a magnetic tape from one of two reels (i.e., the reel on the supply side) and winding the tape on the other reel (on the take-up side), and a control circuit for controlling the operation of the tape transport mechanism. The control circuit comprises remainder detecting means for detecting the amount of the tape remaining unwound and extending to a tape trailing end, speed control means for controlling the speed of movement of the tape based on a tape position corresponding to the detected amount of the remaining tape to bring the tape into an accelerated movement, a constant-speed movement and a decelerated movement successively, and means for estimating the thickness of the tape. The speed control means comprises deceleration starting position adjusting means for altering the tape position where the deceleration of the tape is to be started, in accordance with the estimated thickness of the tape.
With the cassette tape recorder of the invention described above, the remainder detecting means calculates the amount of the remaining tape from the area Ss of a region positioned between and defined by the radially innermost circle of the tape as wound on the reel on the supply side and the radially outermost circle thereof, the area St of a region positioned between and defined by the radially innermost circle of the tape as wound on the reel on the take-up side and the radially outermost circle thereof and the overall length Q of the tape. The tape thickness estimating means calculates the thickness of the tape by dividing the combined area (Ss+St) of the tape wound on the two reels by the overall length Q of the tape. The deceleration starting position adjusting means alters the deceleration starting position toward a leading end of the tape when there is an increase in the estimated thickness of the tape.
For example, if the tape has a thickness greater than the standard value, the true amount of the remaining tape is less than the amount of the remaining tape detected by the remainder detecting means. The tape position (deceleration starting position) where the tape in the constant-speed movement is to be decelerated is therefore shifted toward the tape leading end, whereby the length of tape to be wound by the deceleration movement is adjusted to a predetermined value. Conversely, if the tape has a thickness less than the standard value, the true amount of the remaining tape is greater than the amount of the remaining tape detected by the remainder detecting means. The tape position (deceleration starting position) where the tape in the constant-speed movement is to be decelerated is therefore shifted toward the tape trailing end, whereby the length of tape to be wound by the deceleration movement is adjusted to the predetermined value.
Stated more specifically, the speed control means has a plurality of deceleration slopes which are different in deceleration starting position as speed varying reference curves prescribing the relationship between the tape position and the tape speed during deceleration. The speed control means selects one of the deceleration slopes in accordance with the estimated thickness of the tape, and controls the tape speed based on the selected deceleration slope. With this specific embodiment, the deceleration slopes are prepared in advance for different tape thicknesses, and one of these slopes is selected.
With the tape recorder thus constructed, the speed control means continues the constant-speed movement until the tape speed-position relationship prescribed by the selected deceleration slope is established before a change-over to the deceleration movement. When the tape speed decreases during the constant-speed movement, for example, under an increased load, the speed control means thus adapted continues the constant-speed movement even after the predetermined deceleration starting position is reached, and thereafter initiates deceleration in accordance with the selected deceleration slope when this slope is reached. Thus, deceleration is realized always in conformity with the selected slope regardless of the tape speed of the constant-speed movement.
Further stated more specifically, the speed control means comprises frequency dividing means for frequency-dividing FG pulses in synchronism with the rotation of a motor constituting the tape transport mechanism by N, means for setting the frequency dividing number N, period measuring means for measuring the period of frequency-divided pulses obtained by the frequency division, target period setting means for setting target values in succession for respective periods of frequency-divided pulses, and motor control means for preparing a control signal for the motor based on the deviation of the period measurement of frequency-divided pulses from the target value. At least when the period measurement of frequency-divided pulses is included within a predetermined range, the target period setting means alters an increase in the target period to thereby uniformalize the rate of variation with time of the tape speed during the deceleration movement every time processing is performed for setting the target period and preparing the control signal, whereby the tape is prevented from slackening during the deceleration movement.
Further stated more specifically, the frequency number setting means successively decreases the frequency dividing number N to determine one or a plurality of intermediate frequency dividing numbers and one final frequency dividing number, and frequency-divides the FG pulses by each intermediate frequency dividing number until the number reaches the final frequency dividing number. The period measuring means, the target period setting means and the motor control means execute the foregoing control at each intermediate frequency dividing number. This makes uniform the rate of variation of the tape speed with time from the start of deceleration until termination of deceleration, also realizing control based on the periods of frequency-divided pulses with ease and improved accuracy.
As described above, the cassette tape recorder of the present invention is adapted to shorten the time required for winding up the magnetic tape by providing specified speed variations to the tape in the high-speed winding mode even when the tape thickness varies from tape to tape.